The present invention relates to methods and apparatus for treating a liquid with a gas, and more particularly, to methods and apparatus for efficiently treating wastewater with ozone.
In numerous applications it is necessary to treat a liquid with a gas in order to dissolve the gas in the liquid. Frequently, the liquid will exhibit a demand for a particular gas and by meeting such a demand, certain desired results may be achieved. For example, it is well known to disinfect water by dissolving ozone or chlorine therein for the purpose of providing potable water. Alternatively, it is practical to treat wastewater with ozone in order to aid in reducing the chemical oxygen demand (COD) of the wastewater in a purification process. Realization of the foregoing desired purposes, however, is subject to the implied prerequisite that such processes be conducted economically. Hence, ozone dissolution must be efficient, both in terms of the mechanical energy required and the utilization of supplied ozone.
In order to effectively treat wastewater with ozone, a substantial degree of contacting between ozone gas and wastewater must occur to cause the necessary chemical reaction therebetween. Maintenance of high concentrations of dissolved ozone is also required to effect desirable COD reductions. Previously, plural independent gas-liquid mixing devices have been utilized to obtain required levels of dissolved ozone notwithstanding complexities in controlling such devices in response to varying demands (COD) and flow rates of wastewater for ozone. In addition, as the space available for waste treatment apparatus as aforesaid is generally extremely limited, physically compact treatment systems are preferred. Similarly, the capital cost of such equipment must also be maintained within prescribed limits in order to avoid prohibitively expensive waste treatment systems.
In processes for treating wastewater with ozone, an ozone generator is commonly provided as a source of an ozone enriched feed gas. Overall ozone costs may be reduced by recycling an ozonating gas which has not been consumed by wastewater to the ozone generator.
This is particularly true in applications wherein oxygen is utilized as a carrier gas for ozone. As maximum ozone generation efficiencies are obtainable when oxygen is supplied as the primary starting material, recycling of the oxygen carrier gas is required in view of the cost of producing oxygen. In other applications wherein air is utilized as a carrier gas, recompression and drying of the ozonating gas substantially destroys ozone therein. Generally, this ozonating gas is vented to the atmosphere subsequent to ozone destruction. However, notwithstanding the recycling of an oxygen carrier gas, previous ozone dissolution techniques have required independent control of each of a plurality of gas-liquid mixing devices, which controls are complex and expensive, in order to provide economical treatment processes. Thus, previous ozonation processes have not been efficient in matching the ozone demand of wastewater with the amount of ozone actually supplied to a dissolution device.